1. Field of the Invention
The present invention relates to image processing apparatuses, image processing methods, and non-transitory computer-readable medium.
2. Description of the Related Art
From the viewpoints of economic conditions of recent years and environmental protection, reduction in running cost even of image forming apparatuses for business or personal use has become a focus of attention. For such image forming apparatuses, various types of processing methods have been proposed as a mode for reducing amounts of colorants to be consumed (hereinafter, “colorant usage”) to form (print) an image (see, for example, Japanese Publication No. 2007-50708). There are different types of colorant depending on a type of an image forming apparatus that uses the colorant. The colorant corresponds to toner for electrophotographic recording apparatuses, while the colorant corresponds to ink for inkjet recording apparatuses, stencil printing apparatuses, offset printing apparatuses, and the like. The colorant corresponds to ink ribbons for thermal transfer recording apparatuses. The colorant-usage reducing mode aims at reducing running cost for forming a sheet of an image by reducing amounts of the colorants to be consumed than those of a standard image forming mode in the image forming apparatus. Various methods for forming an image in a colorant-usage reducing mode have conventionally been put forth. Examples of such a method include a first method that multiplies a tone value of each of pixels that form an image represented by image data by a coefficient that is equal to or greater than zero and less than or equal to one for each of cyan (C) colorant, magenta (M) colorant, yellow (Y) colorant, and black (K) colorant to thereby decrease a density of the entire image. The higher the density of an image, the larger amounts of the colorants are required. Accordingly, this method reduces colorant usage by decreasing the density of the entire image. As a second method, a method of reducing colorant usage by applying a mask to an image that is to be output from an image forming apparatus to thereby thin dots that form the image can be employed. A method of gradually reducing dot size is applicable, as a modification of the dot thinning, to an image forming apparatus capable of forming multi-level dots.
As a matter of course, the colorant-usage reducing mode puts a highest premium on reduction of colorant usage and gives secondary concern to image quality. However, unlike former times, occasions of printing colorful, more appealing images containing not only simple graphs but also a number of illustrations and/or pictures taken with a digital camera have increased recently. Printing such an image involves a problem that an image formed in the colorant-usage reducing mode is inferior in image quality to an image formed in a standard image forming mode. This is a problem concomitant to the colorant-usage reducing mode. To make this image degradation obtrusive as little as possible, as a third method, a method of reducing colorant usage by actively performing undercolor removal (UCR) in a black generation (BG)/UCR process so as to maximize a portion of image, in which multiple colors are overlaid on one another, replaced with image data for K-separation has been provided in recent years.
As a fourth method based on a diametrically opposed concept that a minimum requirement for an image is that what is drawn on the image is distinguishable, a method of removing all portions but leaving only outline portions of the image can be employed. This method reduces colorant usage by an amount corresponding to portions inside the outline portions. As a matter of course, an image formed by using this method greatly differs from an image represented by original image data; however, this method makes significant reduction in colorant usage possible. As a fifth method, a method of protecting an object that is preferably not to degrade in image quality by making it possible to select whether to perform image forming in the colorant-usage reducing mode on a per-image-object (a character object, a line drawing object, a graphics object, a picture object, or the like) basis can be employed.
Although the third method or the fifth method allows forming an image in image quality higher than that of the first method and the second method, when an attempt of further reducing colorant usage is made, image quality of an image formed by using the third method degrades to an appreciable level. More specifically, what is drawn in the image can be indistinguishable due to a decrease in image contrast. This tendency becomes more prominent in a picture object or the like object. The fourth method that causes outline portions to be left can be used without problem for a character object or a graphics object, of which shape itself is information. However, as for an object, such as a picture object, for which texture is an important factor, when an image is formed by using the fourth method, disparity in texture stands out, making it difficult to guess an original image.
It is possible to avoid decline in image quality by excluding an object, such as a picture object, with which a trouble is likely to occur, as in the fifth method or by setting a small colorant reducing amount, by which colorant usage is to be reduced, for such an object. However, as can be seen from proliferation of digital cameras and increase in printing of Web pages in recent years, occasions to print images containing a large number of pictures have sharply increased. This can unfavorably decrease effectiveness of the colorant usage reduction by such a method. Furthermore, to switch image processing on a per-object basis, a process of separating objects is required. When an operating system (OS) of a computer has a function of dividing image data into image data pieces each corresponding to an object and outputting the divided image data pieces, image processing can be performed individually on the per-object basis. However, in a situation where the OS does not have such a function, a situation where an image is obtained by scanning with a scanner, or a like situation, it is necessary to perform image-area separation and determine what object each image area is for each of the image areas. Such image-area separation places a very heavy computational load and can result in a decrease in throughput to output of a result of processing, or an increase in price of the apparatus resulting from mounting high-performance processing circuit on the image forming apparatus. Furthermore, even when image-area separation is performed, object(s) that exists in an image is not always recognized appropriately, and hence it is necessary to keep taking possibility of erroneous recognition into consideration. Accordingly, intended image quality is not always achieved.
Under such circumstances, forming a high-quality image with colorant usage reduced by a large amount but with restricted degradation in image quality has been desired.